Re: [patch 1/2] x86_64 page fault NMI-safe

[Ingo Molnar <mingo@elte.hu>]

* Dave Chinner <david@fromorbit.com> wrote:

> On Tue, Aug 03, 2010 at 11:56:11AM -0700, Linus Torvalds wrote:
> > On Tue, Aug 3, 2010 at 10:18 AM, Peter Zijlstra <peterz@infradead.org> wrote:
> > >
> > > FWIW I really utterly detest the whole concept of sub-buffers.
> > 
> > I'm not quite sure why. Is it something fundamental, or just an
> > implementation issue?
> > 
> > One thing that I think could easily make sense in a _lot_ of buffering
> > areas is the notion of a "continuation" buffer. We know we have cases
> > where we want to attach a lot of data to one particular event, but the
> > buffering itself is inevitably always going to have some limits on
> > atomicity etc. And quite often, the event that _generates_ the data is
> > not necessarily going to have all that data in one contiguous region,
> > and doing a scatter-gather memcpy to get it that way is not good
> > either.
> > 
> > At the same time, I do _not_ believe that the kernel ring-buffer code
> > should handle pointers to sub-buffers etc, or worry about iovec-like
> > arrays of smaller ranges. So if _that_ is what you mean by "concept of
> > sub-buffers", then I agree with you.
> > 
> > But what I do think might make a lot of sense is to allow buffer
> > fragments, and just teach user space to do de-fragmentation. Where it
> > would be important that the de-fragmentation really is all in user
> > space, and not really ever visible to the ring-buffer implementation
> > itself (and there would not, for example, be any guarantees that the
> > fragments would be contiguous - there could be other events in the
> > buffer in between fragments).  Maybe we could even say that fragments
> > might be across different CPU ring-buffers, and user-space needs to
> > sort it out if it wants to (where "sort it out" literally would mean
> > having to sort and re-attach them in the right order, since there
> > wouldn't be any ordering between them).
> > 
> > From a kernel perspective, the only thing you need for fragment
> > handling would be to have a buffer entry that just says "I'm fragment
> > number X of event ID Y". Nothing more. Everything else would be up to
> > the parser in user space to work out.
> 
> Heh. For a moment there I thought you were describing the the way XFS writes 
> transactions into it's log. Replace "CPU ring-buffers" with "in-core log 
> buffers", "userspace parsing" with "log recovery" and "event ID" with 
> "transaction ID", and the concept you describe is eerily similar. That 
> includes the fact that transactions are not contiguous in the log, can 
> interleave fragments between concurrent transaction commits and they can 
> span multiple log buffers, too. It works pretty well for scaling concurrent 
> writers....

That's certainly a good model when you have to stream into a 
persistent-storage transaction log space with multiple writers.

The difference is that with instrumentation we are generally able to make 
things per task or per cpu so there's no real multi-CPU 'concurrent writers' 
concurrency.

You dont have that luxory/simplicity when logging to storage, of course!

Thanks,

	Ingo